1. Field of the Invention
The present invention relates generally to the field of circulating fluidized bed (CFB) reactors, combustors or boilers and, in particular, to a simple system which permits recycling of the finest particles that would otherwise be carried over with the gas flow exiting a separator used in combination with such CFB reactors, combustors or boilers. The invention thus permits enhanced utilization of reagents in such CFB equipment.
2. Description of the Related Art
A necessary condition for efficient utilization of reagents in a CFB reactor, combustor or boiler, such as combustion of fuel and/or sulfation of sorbent in a CFB boiler, is reagent particle circulation in the unit providing sufficient residence time for reactions to complete. This is achieved by solids separation from gases leaving the reactor and recycling these solids back to the unit.
The solids recycle systems may be single-stage or multi-stage. For a CFB boiler, as shown in FIG. 1, a single-stage system typically includes a cyclone separator 10 located downstream of the furnace 20 and a solids recirculation loop comprised of a standpipe 30 connected to a lower portion 35 of the cyclone 10 and a non-mechanical device 40 to seal against gas by-passing the separator. For example, the non-mechanical device 40 can be a syphon valve.
A double-stage system may include two cyclones connected in series (not shown in the Figures), each having its own recirculation loop, or an impact-type particle separator 11, as shown in FIG. 2. The impact-type particle separator 11 is typically an array of U-shaped beams or similarly shaped elements arranged at the furnace exit 21. A secondary particle collection device 31 is positioned after the impact-type particle separator 11 (downstream with respect to a flow of gases and entrained particles through the CFB reactor). A common secondary particle collection device 31 is a mechanical dust collector, such as a multicyclone or multiclone dust collector (MDC). In this type of system, the bulk of the solids leaving the furnace 20 are collected and recycled by the primary stage particle separator 11, while the secondary stage collects and returns most of the fine particles passing through the primary particle separator 11 back to the furnace 20.
The CFB process could benefit if the above-identified particle separation/collection devices were more effective in collecting fine particles from the flue gases. The effect is that fewer fine particles are recycled prior to leaving the CFB unit, and thus less time is available for reaction of the particles. Although fine particles require less reaction time, the majority of unreacted material exiting the system, such as unburned carbon and unsulfated sorbent in CFB boilers, is concentrated in the finest particles. These fine particles usually have diameters below 50-70 microns.
Fine particles of this size are commonly collected in a baghouse or electrostatic precipitator. U.S. Pat. No. 5,343,830 to Alexander et al. discloses one recycling method which recycles the fine particles collected in the baghouse or electrostatic precipitator back to the reactor. flowever, this method requires installation of a complex solids recycle system.
Any notable improvement of fine particle collection in the cyclone or any other inertial-type separation device presently known using increasing swirling and outlet gas velocity, if possible, will result in a prohibitively high pressure drop and increased parts wear.
Alternatively, a mechanical dust collector can be used to increase fine particle collection, as taught by a Russian publication, Aerodynamic Calculation of Boiler Units (Standard Method), Edited by S. I. Mochan, 3.sup.rd Ed., Leningrad, "Energia", 1977. As shown on page 87 thereof, gas is pulled out from the mechanical dust collector ash hopper and recycled back to the mechanical dust collector inlet using a dedicated fan. The recycle gas stream is cleaned of ash using high efficiency cyclones placed in the recycle loop.
Gas flow pulled from the separator in the same direction as collected solids entrains some the finest particles that otherwise would be carried over with the gas flow leaving the separator, thus, improving collection efficiency for those particles. This method does not cause a gas velocity increase in the collecting elements outlet pipes that normally contributes to a large share of the elements' pressure drop and erosion potential.